H there! My name is***** am sorry that you are having this problem. I will do my best to help. I will need to know what brand of e-reader you have? For example, is it a Kindle or a Nook, or something else?
How are you trying to add your Yahoo account? Are you doing something like this?
What exactly happens when you try to add your email account? Is there an error message? If so what does it say?